YouTube, meanwhile, said that it has made progress in addressing the issue.

"We've been conducting an extensive review of our advertising policies -- and in recent weeks have made significant changes that give brands more control over where their ads appear," a spokesperson said.

YouTube sells ad time to companies, and their spots are slotted to play before videos. Advertisers don't directly control where their ads are placed, although they can specify the demographics they'd like to target.

YouTube relies heavily on viewers to flag content. With 300 hours of content a minute uploaded, some fall through the cracks.

The company said that while "no system will be 100% perfect," it has taken steps to "further safeguard our advertisers' brands."

The latest round of advertising suspensions included three major political parties in the U.K. The country will hold a general election on Thursday.

The Times reported that the Conservative Party and the Liberal Democrats had asked for their ads to be removed from the site. The Labour Party confirmed it had taken similar action.

"We will be raising this issue with YouTube and have paused all adverts until we are satisfied Google have taken steps to make sure this does not happen again," the party said in a statement.

In March, Google(GOOGL) faced an advertiser exodus after spots from major brands and the U.K. government ran alongside content from extremist groups.

At the time, the British government summoned the tech firm to explain why taxpayer-funded ads were used on content including videos from former Ku Klux Klan leader David Duke.

Google and Facebook(FB) own huge chunks of the digital advertising business, and major brands are spending an increasing share of their ad budgets on their platforms.

Companies are becoming cautious about where their advertisements appear, however. Vodafone(VOD) announced plans on Tuesday to prevent its ads from running on sites that carry hate speech and fake news.